C/EBP is a member of the bZIP class of transcription factors and has been implicated in a variety of cellular processes involving gluconeogenesis. We are interested in unraveling its role in fat differentiation but "knock out" mice die soon after birth. To examine C/EBP function in fat, we have expressed a dominant negative (D-N) to C/EBP under the control of a fat specific promoter. Analysis of these mice is presently under way but initial examination indicates the mice have no fat. To create a D-N to C/EBP, we replaced the normal basic region with a designed acidic region that forms a coiled coil with the basic region and stabilizes the complex 1,000 fold. This D-N prevents DNA binding in an equimolar fashion and the inhibition of C/EBP transactivation is dependent on both the C/EBP leucine zipper and the acidic extension. This general strategy can be shown to work for all the bZip and bHLHZip proteins we have examined including the oncogenes Fos/Jun and Myc/Max. These studies indicate we are able to inhibit DNA binding of both classes of transcription factors in a dimerization dependent fashion. We are presently using the tetracycline induceable system to regulate the expression of the C/EBP dominant negative in a differentiating fat cell line and in transgenic mice.